This invention relates, in general, to resonator devices, and more particularly to monolithic resonators which are integrated with other electronic devices as part of a monolithic integrated circuit.
In the design of radio receivers, particularly paging receivers, cellular radios, and microwave satellite communication systems, it is desirable for components which form the system to take up as little space as possible. It is desirable for as many components as possible to be integrated into a single integrated circuit. This integration also reduces the number of connections needed to make the radio, greatly improving reliability and reducing manufacturing cost.
One problem that is basic to the operation of a high frequency radio, however, is generation of a stable high frequency oscillating electric signal which is used to both transmit and receive information. Another basic problem is the separation of one signal from another by means of resonant filters. Both problems require bulk structure resonators with low signal loss, a very well defined resonance point, and excellent frequency stability. Usually quartz crystals are used to make high frequency resonators. The technology of quartz crystal oscillators limits their ability at extremely high or extremely low frequencies. For frequencies above about 300 Mhz a thin film non-conductive piezoelectric resonator is commonly used, and for frequencies below approximately 100 Khz a non-piezoelectric mechanical resonator is commonly used.
It has been known for some time that certain crystalline materials have piezoelectric properties. Specifically, there is what is called a direct piezoelectric effect, in which electrical charges appear on crystal surfaces upon the application of an external stress. There is also a converse piezoelectric effect, in which the crystal shows strain or deformation when an electrical charge is applied by external means to faces of the crystal. These effects have been used for many years in quartz crystal resonators and other devices in which bulk acoustic waves are transmitted through a crystal, typically between electrode plates at opposite faces of the crystal. These quartz crystal resonators are called bulk resonant devices because the vibration is propagated throughout the bulk of the crystal. Another form of piezoelectric resonator confines the bulk vibration to the surface as an acoustic surface wave induced into a thin film of piezoelectric crystal, and is known as a surface wave device.
Until now however, devices which use an unsupported piezoelectric layer have not been available. Since silicon is not a piezoelectric material, electroacoustic devices could only be made by forming a piezoelectric layer on top of a micromachined silicon structure. Mechanical coupling between the piezoelectric film and a non-piezoelectric material results in damping of the acoustic wave and lowered quality factor (Q) of acoustic wave filters and oscillators. As a result the bulk structure resonators have not been integrated with other components, and so must be coupled to other components on PC boards or hybrid substrates.
Another form of bulk structure resonator uses a physical vibration of a non-piezoelectric structure at a resonant frequency. Typically a non-piezoelectric mechanical resonator of this type is constructed in the same way as a tuning fork or a spring operated pendulum, with a known mass held at the end of a spring and vibration is induced throughout the bulk of the structure by externally applied forces. Techniques are known to micromachine silicon structures to form diaphragms, bridges, and cantilever beams which can then oscillate when a bulk vibration is established in them. These micromachined structures allow higher frequency operation than discrete structures because of the smaller geometries used.
The advantages of monolithic integration are numerous. Compared with a monolithic integrated system, one which requires multiple components will: require more space, be less reliable, have a higher power consumption, and less precise temperature control, have a lower shock tolerance, have greater stocking and spares distribution problems aggravated by the need for high accuracy oscillators to have a matched resonator and oscillator circuit, and will have higher costs at every stage of design and production.
Clearly there is a need for a resonator which can be monolithically integrated with other semiconductor devices, in which the bulk structure resonator is not in contact with anything which would inhibit vibration. This element must be capable of fabrication as a non-piezoelectric mechanical resonator, a quartz crystal resonator and as a piezoelectric film resonator to provide a wide range of operating frequencies.